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Energy stored in spring is given by E = (kx²)/2
therefore spring constant , k = 2.E /x²
E is the energy stored by spring = 72 J
x is the Displacement from equilibrium = 12 cm or 0.12 m
substituting values we get k = 72* 2 /(0.12^2) = 10000
Hence Spring constant 10000.0 kg s⁻² (Answer)
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